Differentials may be used on vehicles to distribute torque between front and rear axles or between left and right axle shafts on a vehicle. However, under some driving conditions, for example during conditions likely to cause wheel slip or during periods of wheel slip, it may be advantageous to lock the rear and front axles together, or to lock the right axle shaft to the left axle shaft (of either the rear or front wheels). Differentials that utilize dog clutches are known in the art to provide such locking arrangements.
Ideally, when using a dog clutch differential to lock shafts, an operator will engage the differential prior to slip or bring the vehicle to a stop prior to engaging the dog clutch. In practice this may not always happen, especially if the wheels of the vehicle begin to slip prior to engagement of the dog clutch differential. Engagement of the dog clutch differential, when there is excessive motion between the two members (such as during tire slip), may cause ratcheting or grinding of the teeth of the dog clutch and may ultimately result in damage to the dog clutch differential or other components.
U.S. Pat. No. 7,195,579, issued Mar. 27, 2007, is an example of prior art in the actuation of dog clutch differentials in which a microprocessor momentarily communicates a message to an engine electronic controller to brake engine torque when the sliding clutch and helical gear of a dog clutch are placed in engagement mode or disengagement mode. While beneficial for conventional transmissions in which a torque converter is coupled to a mechanical power shift, this arrangement may not reduce torque output for a Continuously Variable Transmission (CVT) because CVTs typically receive a speed or torque command that is independent of engine speed or torque. Thus, simply reducing engine speed or torque on a CVT application could cause an undesired engine lug or stall rather than the desired reduction in CVT output torque. A better method and system is needed that can be used on CVTs that are controlled independently of engine speed.